Melanocortins are neuropeptides that arise from pro-opiomelanocortin (POMC), which is most prevalently expressed in the arcuate nucleus of the hypothalamus, pituitary lobes, and the nucleus tractus solarius of the brainstem. [Gantz, I., et al., Molecular Cloning, Expression, and Gene Localization of a Fourth Melanocortin Receptor, J. Biolog. Chem., 1993, 268, 15174–15179.] These peptides include ACTH, α-MSH, β-MSH, γ1-3-MSH, and synthetic analogue NDP-αMSH (Wikberg, J E S, Melanocortin receptors: new opportunities in drug discovery, Exp. Opin. Ther. Patents, 2000, 11(1), 61–76).
These peptides bind to five types of melanocortin receptors (MC1-MC5), which are G-protein coupled receptors that all positively modulate adenylate cyclase. The MC4 and MC5 receptors are widely distributed in the brain and spinal cord, whereas the MC3 receptor is located mainly in the hypothalamus. [Gantz, I., et al., supra.] The MC4 receptor is selectively activated by αMSH and can induce neurite outgrowth in Neuro 2A cells. (Adan R. A. H, et al., Molecular Brian Research, 1996, 36, pp 37–44; Mountjoy, K. G., Mortud, M. T., Low, M. J., Simerly, R. B. and Cone, R. D., Mol. Endocrinol., 1994, 8, pp 1298–1308). ACHT is a less potent activator of the MC4 receptor than αMSH. (Adan, R. A. H., Cone, R. D., Burbach, J. P. H. and Gispen, W. H., mol. pharmacol., 1994, 46, pp 1182–1190). The MC5 receptor is activated, in order of degree, by NDP ≈α-MSH>ACHT (1–24)≧α MSH ACHT (1–39)=βMSH>>γMSH (The Melanocortin Receptors, Cone, R. D., Editor, Human Press Inc., Totowa, N.J., 2000, Chen, W., pp449–472)
In whole animals, studies in the rat sciatic nerve crush model have demonstrated that α-MSH increases neurite outgrowth and, as the most potent of the ACTH derived peptides, it significantly promotes nerve terminal branching, endplate area, and perimeter. [Bijlsma, W. A., et al., The Enhanced Recovery of Sensorimotor Function in Rats is Related to the Melantropic Moiety of ACTH/MSH Neuropeptides, Eur. J. Pharmacol, 1983, 92, 231–236; Van der Neut. R., et al., Stimulation by Melanocortins of Neurite Outgrowth from Spinal and Sensory Neurons In Vitro, Peptides, 1992, 13, 1109–1115; Van Der Zee, C. E. E. M., et al., α-MSH and Org 2766 in Peripheral Nerve Regeneration: Different Route of Delivery, Eur. J. Pharmacol., 1988, 147, 351–357; Strand, F. L., et al., Melanocortins as Factors in Somatic Neuromuscular Growth and Regrowth, Pharmac. Ther., 1994, 62, 1–27]. Furthermore, recovery of motor function after nerve injury is shortened by application of α-MSH and other melanocortins. [Strand, F. L., et al., supra]
Mice in which the MC4 receptor is rendered inactive by gene targeting become obese, suggesting that the MC4 receptor is involved in feeding. [Huszar, D., et al., Targeted Disruption of the Melanocortin4 Receptor Results in Mice, Cell, 1997, 88, 131–141] This is substantiated by a report that various MC4 peptide agonists inhibit feeding behavior in agouti mice. [Fan, W., et al., Role of Melanocortingenic Neurons in Feeding and the Agouti Obesity Syndrome, Nature, 1997, 385, 165–168]. α-MSH induces grooming behavior in rats, but the significance of this is not clear and may not be mediated via the MC4 receptor. [Adan, R. A. H., et al., Differential Effects of Melanocortin Peptides on Neural Melanocortin Receptors, Molecular Pharmacology, 1994, 46, 1182–1190].
The melanocortins αMSH and ACTH are also known for their ability to stimulate pigmentation and adrenal glucocorticoid secretion, respectively. The role of melanocortins, particularly αMSH, in the regulation of sebaceous gland activity (an exocrine gland with holocrine type of secretion) was shown originally in rats. More particularly, the studies showed that removal of the intermediate lobe of the pituitary (which produces the POMC peptides) resulted in decreased sebaceous lipid production, with complete restoration to normal levels after replacement therapy with αMSH (Thody, A. J. and Shuster, Nature, 237, 346–347, 1972). In a study of rats following total hypophysectomy, treatment with αMSH resulted in an increase of sebum production, although full restoration of sebum production was achieved only after treatment with a combination of αMSH and testosterone (Thody, A. J., Shuster, S., J. Endocr. 64, 503–510, 1975; Ebling, F. J., Ebling, E., Randall, V. and Skinner, J., J. Endocr. 66, 407–412, 1975). Knock-out mice where the MC5 receptor was deleted were observed to display a severe defect in water repulsion and thermo-regulation, due to decreased production of sebaceous lipids (Chen, W. Kelly, M. A., Opitz-Araya, X., Thomas, R. E., Low, M. J., and Cone, R., Cell, 91, 788–798, 1997).
The MC5 receptor is known to be expressed in human sebaceous glands, and may be involved in the regulation of human sebaceous lipid synthesis. Human MC5-R has been cloned and characterized (Chhajlani, V., Muceniece, R., Wikberg, J E S., Biochem. Biophys. Res. Commun. 195, 866–873, 1993). Moreover, presence of MC5-R m RNA in human sebaceous glands has been shown by RT-PCR and the protein was detected by immunohistochemistry and Western blot analysis (Thiboutot, D., Sivarajah, Gililand, K., Cong, Z. and Clawson, G., J. Invest. Dermatol. 115(4), 614–619, 2000).
Human sebum differs in its composition from other mammals. The main lipids in human sebum are triglycerides, wax esters and squalene (Greene, R. S., Downing, D. T., Poci, P. E., Strauss, J. S., JID 54, 240–247, 1970). Squalene, for instance is not found in many mammals with the exception of otter and beaver. Triglyceride, which is a major component of human sebum is poorly represented in other species and in many (e.g. chimpanzee) appears to be totally absent (Thody, A. J., Shuster, S., Physiolog. Rev. 69, 383–415, 1989). Moreover melanocortins can have different effects on cells from different species. For example both αMSH (EC50=3.7 nM) and ACTH (EC50=16.4 nM) are potent lipolytic agents for rabbit adipocytes, whereas in the rat only ACTH (EC50=1.34 nM) has potent lipolytic activity (Ramachadran, J., Lee, V., 428, 339–346, 1987; Richter, W. O., Schwandt, P., Neuropeptides 9, 59–74, 1987). Despite lipolytic activity in rodents and rabbits, ACTH has very little effect on lipolysis in isolated human and non-human primate adipocytes, even at concentrations as high as 1 μM (Ng, T. B. Comparative Biochem. 97, 441–446, 1990). Thus defining the role of melanocortins and their receptors in animal sebaceous model systems is not necessarily predictive of their role in a human sebaceous lipid regulation.
Recently, Basu et. al., in WIPO publication WO99/55679 disclosed isoquinoline derivatives, small molecule non-peptide compounds, which showed low micromolar affinities for the MC1 and MC4 receptors, reduction of dermal inflammation induced by arachidonic acids, and reductions of body weight and food intake.
Nargund et. al., in WIPO publication WO99/64002 disclosed spiropiperidine derivatives as melanocortin receptor agonists, useful for the treatment of diseases and disorders such as obesity, diabetes and sexual dysfunction.
Thus there exist a need for small molecule modulators of the melanocortin receptor, more particularly the melanocortin-3, melanocortin-4 and/or the melanocortin-5 receptors.